I. Field of the Invention
This invention relates generally to the field of energy storage devices. In particular, the present invention relates to the controlled deposition of smooth and conformal thin metal films on high-surface-area carbon nanostructures. This invention further relates to the use of these nanostructures for supercapacitor electrodes.
II. Background of the Related Art
In its simplest form, a capacitor is an energy storage device comprised of two conducting plates separated by an insulating layer. When a voltage is applied to the plates, positive and negative charges are induced on opposite surfaces and an electric field is generated. The ability of a capacitor to store electrical charge is defined as its capacitance which is directly proportional to the polarizability of the insulating layer and the surface area of the plates, but is inversely proportional to the separation between the plates. Thus, the larger the plate surface area, the greater the polarizability of the insulating medium; and, the smaller the plate separation, the greater the resulting capacitance.
Batteries are another type of energy storage device which generally produces electrical energy by the oxidation and reduction of electrochemical reagents within the battery. In this case the energy storage and conversion process is Faradaic since electron transfer between the electrodes occurs. Charge storage in capacitors is generally non-Faradaic since the storage of electrical charge is fully electrostatic with no electron transfer occurring across the electrode interface. While batteries are capable of attaining high energy densities over a wide range of voltages, they cannot attain high power densities and can only undergo a limited number of recharge cycles. Capacitors can provide high energy transfer rates with a nearly unlimited number of recharge cycles, but have limited charge storage capabilities.
Advances in energy storage devices eventually led to the development of the electric double-layer capacitor which is also known as an electrochemical capacitor or supercapacitor. A supercapacitor is an electrochemical energy storage device which combines the high energy storage capabilities of a battery with the high power and nearly unlimited recharging cycles attainable with a capacitor. A comprehensive review of the development and operation of supercapacitors is provided by B. E. Conway in “Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications,” Kluwer Academic/Plenum Publishers, NY (2003) the entire contents of which is incorporated by reference as if fully set forth in this specification.
A supercapacitor is generally comprised of opposing porous, yet polarizable electrodes which are interspersed with an electrolyte and separated by an exceedingly thin, yet insulating and porous separator layer. The electrodes themselves are typically comprised of a porous material having a very large surface area. This assembly is situated between two opposing current collectors, each of which is in contact with an outer surface of an electrode. The exceedingly thin separator layer combined with the large surface area of the electrodes yields a device with an extraordinarily high capacitance.
Nanoporous electrode materials such as graphite, carbon fibers, charcoal, vitreous carbon, carbon aerogels, and activated carbon have previously been employed as supercapacitor electrodes. Factors which affect the charge storage efficiency of such carbon-containing electrodes include the availability of surface area for the accumulation of charge, electrolyte accessibility to intrapore surfaces, electrical conductivity within porous matrices, as well as the chemical stability and electrical conductivity of the electrode itself. Activated carbon is commonly employed as the electrode material due to its relatively large specific surface area which is on the order of 1000 to 2000 m2/g. However, its small pore size (typically a few nm in diameter) makes it difficult for ions in the electrolyte to access intrapore surfaces. Furthermore, the use of insulating polymeric binders to fabricate the electrodes is detrimental to performance since it increases the resistance of the electrode.
Some of the problems associated with activated carbon may be circumvented by using carbon nanotubes as the electrode material. Carbon nanotubes are nanometer-scale cylindrical structures comprised entirely of sp2 bonded carbon atoms. Although the specific surface area of carbon nanotubes may be considerably lower than that of activated carbon or carbon fiber, electrodes with a higher capacitance per unit surface area and lower internal resistance can be obtained. This is due primarily to the larger pore structure of carbon nanotube aggregates which permit greater accessibility to the available surface area. However, access to inner wall surfaces of nanotubes is inhibited by the small diameter of the tube ends and its proportionally larger length.
A still higher capacitance may be obtained using carbon nanohorns which have a structure analogous to nanotubes, but with one end of the cylindrical tube closed and the other open, resulting in a horn-like shape. Since carbon nanohorns have a more open structure, both the internal and outer surfaces of carbon may be made accessible to adsorbates. Consequently carbon nanohorns generally possess a higher specific surface area than carbon nanotubes and an average pore size (on the order of tens of nm) which is larger than both carbon nanotubes and activated carbon or carbon fibers.
From among available metal electro catalysts, ruthenium (Ru) exhibits the most potential for improving the storage capability because of its multivalent states which permit greater charge storage through an oxidation reaction wherein Ru→Ru4+. Furthermore, Ru remains adsorbed on the surface even after undergoing a change in oxidation state. The utilization of Ru is, however, inhibited by the high cost and scarcity of Ru as well as the toxicity of its oxides. Controlled deposition of smooth, conformal thin films of Ru in the submonolayer to multilayer thickness range is also difficult to achieve. This is primarily due to the tendency of Ru to form films having a high surface roughness with granular nanoparticles dispersed across its surface.